Opioids are the mainstay for treatment of postoperative pain in children. Experience dictates that opioids have narrow therapeutic indices and large inter-patient variability in response. This leads to serious side effects like respiratory depression in up to 50% of children undergoing invasive surgery, which can be fatal. Moreover, up to two-thirds of the inter-individual variability in response to opioids results from genetic variations both from variability in pain perception as well as response to the pain medicine. Much of the inter-individual variability in response to opioids following surgical procedures can be explained by Single Nucleotide Polymorphisms (SNPs) in a subset of the genes that encode proteins involved in pain perception, opioid transport/metabolism (pharmacokinetics), and opioid receptor signaling (pharmacodynamics). It is evident that there are particular children who are more susceptible to suffering side effects and having inadequate pain relief from opioids. Identifying genetic and non-genetic predictors for this susceptibility is vital for safe and effective analgesia in children. This is a critical knowledge gap in medical literature that significantly impacts pediatric pain management. The investigators' central hypothesis is that specific genetic polymorphisms in genes involved in pain perception, opioid transport, and opioid receptor signaling pathways contribute significantly to pain sensitivity, opioid side-effects, and analgesic efficacy in children.